System Component Design Review Compressor Selection

Compressor selection is probably the single greatest influence on power consumption costs in a refrigeration system design. With large numbers of competent “engineering contractors” providing refrigeration services to the industry, it is occasionally disenchanting to see competition “force” end users into purchasing systems which could easily carry a 10% to 15% operating cost penalty because of compressor selection (sizing and type). For years refrigeration systems were distinctly characterized by either single stage for high temperature applications, or two stage for lower temperature applications. Up until the late 1960’s/1970’s, the use of high compression ratio reciprocating machines, such as Howe and Frick’s HDI series and Kohlenberger’s on the west coast, were wide spread.

Generally, two stage systems were provided with either compound piston machines, or two stage systems using rotary boosters for the first stage of compression and piston machines on the second stage. In the last twenty years, the use of screw compressors has had a profound effect on the application of compressor types and sizes. Screw compressors undoubtedly marked the end of the high compression ratio 10 to 12 to 1 (Frick HDI type compressor).

With its lack of discharge valves and use of oil cooling and liquid injection, the screw compressors can provide for a wide variety of compression applications. This has influenced the compressor selection process as much as any single event in refrigeration compression technology in the last 100 years. We are, however, still learning how to use this technology for its best useful purposes.

An emergence of a second technology has somewhat benefited, however complicated and slowed the learning curve, in proper application of screw compressors. This is the advent of PC controllers or microprocessor control of screw compressors. The compressor industry is probably looking at its fourth and fifth generation of microprocessor control of screw compressors, and we are looking at the second and third generation of refrigeration systems and process controls with undoubtedly many more to come.

In the past, the use of pressure switches for compressor cycling and step controllers, such as Vilter’s “Unimatic” or the more generic “Barber Coleman” type of controllers, have certainly provided good control methods. However, these are far less versatile than programmable type controllers using pressure transducers. Very often, the perception conveyed when a PC control system is used for refrigeration control or energy management is that a large amount of energy has been eliminated.

In reality, this is often not the case, particularly in screw compressor applications. We have seen many installations with three or more suction requirements having compressors which average less than 75% full load. A screw compressor will use 25 to 50% more power (KWH) consumption than in its 100% load position. (Note: We have measured consumption by screw compressors with “Journal Bearings” in lieu of Ball Bearings to be as much as 36% more than a reciprocating compressor when the screw compressor was fully loaded.) All to say, a 400 HP screw averaging 75% unload could easily consume 50% more power which is 1,752,000 KWH per year wasted. At $.05/ KWH, this equals $87,600/year. Obviously, this is enough to have justified another, perhaps smaller, compressor.